It is well known to secure one or a plurality of teeth by means of an implant osseointegrated into the alveolar bone of a patient. One commercially available system, known as the Branemark System (Trade Mark), is shown in FIG. 1 of the accompanying diagrammatic drawings. Referring to FIG. 1, the system includes a titanium fixture 2 which is externally screw-threaded and which includes an inner channel 4 which is also screw-threaded. The fixture 2 is positioned within a threaded bore, formed in the alveolar bone, and allowed to osseointegrate over a period of about six months. In due course, an abutment sleeve 6 is positioned on the fixture 2 and secured thereon by means of an abutment screw 8. The abutment sleeve 6 extends through the mucosa above the alveolar bone and an upper part of the abutment screw 8 extends therefrom. The abutment screw 8 includes a tapered neck portion 10 which is arranged to provide a position of location for a correspondingly tapered portion of a gold cylinder 12. When assembled in a patient's mouth, an impression of the arrangement is taken and subsequently a prosthesis 13 which includes a gold framework 14 arranged to co-operate with the gold cylinder 12 and which carries an acrylic tooth 16 is made.
The prosthesis 13 may be positioned by locating the gold cylinder 12 and gold framework 14 upon the neck portion 10 of the abutment screw 8, and then securing the framework in position using a gold screw 20 which is received in internally threaded bore 22 of the abutment screw 8.
A problem associated with the use of the Branemark System as described above is that of the precise location of the tapered portion of the gold cylinder 12 (and tooth 16) with the tapered portion of the abutment screw 8, in order to ensure that any load upon the tooth 16, for example, during mastication, is equally distributed. This problem is particularly acute when a plurality of abutment screws 8 are located at spaced apart positions in the alveolar bone of a patient and wherein gold cylinders 12 are provided for co-operation with a single framework, carrying a plurality of teeth, and being arranged to extend between the abutment screws 8. It has been found in practice that, in many instances, there is an unequal distribution of loads on the abutment screws 8, leading to undue stresses upon specific regions of alveolar bone and also the loosening of the prosthesis. It is also noted that the use of components made of gold in the system described add significantly to the cost of such dental implants.
Dental implant devices have been proposed with a view to equally distributing loads upon the implant and providing an implant of lower cost. For example, U.S. Pat. No. 4,767,328 (Branemark) discloses a device for securing a plurality of teeth, comprising components 2, 4, 6, 8 and 10 described above. The device further includes a prefabricated rigid splint having an arc shape to fit the curve of the alveolar bone, and being attachable to a plurality of spaced apart abutment screws 8. An assembly comprising a plurality of false teeth is removably attached to the splint and the splint is arranged to function as a force distribution means during use. Optionally, a resilient member may be positioned between the assembly comprising the false teeth and the splint in order to further distribute the load. Nonetheless, with the device, it is still necessary to very precisely fabricate the splint for co-operation with the tapered portions of the abutment screws 8.
More recently, U.S. Pat. No. 5,057,017 (Sillard) has proposed a fixed removable dental implant system which includes components 2, 4, 6, 8 and 10 as described above and a precision cast bar securable to a plurality of abutment screws 8 by means of alloy screws. The construction of the precision cast support bar is accomplished using an impression of the patient's mouth from which a mould is made. The precision cast support structure is cast in a vacuum so as to reduce flaws thereof. Again, however, it is necessary to very precisely fabricate the support bar for co-operation with the tapered portions of the abutment screws 8.
It is an object of the present invention to alleviate some of the problems associated with known teeth securement systems.